Device for a ball-and-socket joint type connection of two members

ABSTRACT

The invention relates to a device for a ball-and-socket-type connection of two members. Said device including a first member and a second member, which are interconnected by a ball-and-socket-type articulation; the first articulation segment is configured on the first member as an articulation head and the second articulation segment is configured on the second member as a cavity that receives the articulation head; the cross-sectional surface of the cavity being tapered at the lower end of the second member as a result of a constriction; at least one of the articulation segments in the contact area of the latter is configured spherically with a center of articulation Z; the ball-and-socket-type connection between the two articulation segments can be detachably fixed by means of a clamping element that can be connected to the second articulation segment; the articulation head has a recess, in which the front end of the clamping element can rest; the recess and/or the front end of the clamping element has an at least partially spherical surface with a centre C; and in the clamped position of the device, the center of articulation Z and the centre C are identical.

FIELD OF THE INVENTION

The invention relates to a device for the ball-and-socket joint typeconnection of two parts.

BACKGROUND OF THE INVENTION

A stabilization device with interconnects the bone-fixation means, suchas pedicle screws, with a longitudinal member, such as a longitudinalspinal rod, extending NYJD: 1531363.1 along at least a portion of apatient's spinal column, is used, for example, for the internalstabilization of the spinal column. In order to securely stabilize thespinal column, it must be possible to connect the bone-fixation means,on the one hand, firmly with the vertebrae and, on the other, with thelongitudinal member. Preferably, the head of the bone fixation means andthe longitudinal member are connected by a connecting mechanism, whichpermits the longitudinal member to be fixed with respect to thebone-fixation means at a variety of different angles. Moreover, theconnecting mechanism may be detachable, so that the mechanism may belater removed without large openings be formed in the tissue in theregion of the spinal column.

Such a connection mechanism between a bone-fixation means and alongitudinal member for the internal stabilization of a vertebrae isknown, for example, in U.S. Pat. No. 5,466,237 to Byrd which discloses abone-fixation screw having a screw head, which is constructed with aspherical shaped lower surface and a convexly shaped upper surface. Thespherical shaped lower surface is mounted in a borehole formed in theconnecting mechanism, this borehole comprising a concave section, whichtapers towards the screw shaft, as a result of which, and together withthe screw head, a ball-and-socket type joint connection is establishedbetween the bone-fixation screw and the connecting mechanism. Thisconnection is locked by tightening a nut, which can be screwed over anexternal thread formed on the upper portion of the connecting mechanism.Tightening the nut, forces the nut against the longitudinal member,which is placed in a transverse channel formed in the connectingmechanism for this purpose. The longitudinal member thereafter presseson the convexly-shaped upper surface of the bone-fixation screw, sothat, as the nut is tightened, the longitudinal member and thebone-fixation screw are locked in the connecting mechanism.

A bone-fixation screw having a screw head with a spherically shapedlower surface is also disclosed in International Publication No. WO01/03593 to Frigg which discloses that the center of the spherical lowersurface and the spherical upper surface can be spaced apart or coincidewith respect to the longitudinal axis of the bone-fixation screw.

An another device for connecting a bone-fixation screw with alongitudinal member, which permits limited, separate locking of thelongitudinal member and the bone-fixation screw, is disclosed in GermanPublication No. DE 43 07 576 to Biedermann.

One disadvantage of the connecting mechanism in these known devices isthat the connecting mechanism must have a generally large, overallheight.

A device for connecting a bone-fixation screw with a longitudinalmember, which permits a lower overall height due to the configuration ofthe joint connection and its locking mechanism, is disclosed inInternational Publication No. WO 01/22893 to Lombardo which discloses abone-fixation screw having a screw head with a partially lower sphericalsurface and a top surface having a coaxial, conical cavity for receivinga convexly formed front end of the clamping means which accommodatespolyaxially movement and which may be pressed by mounting means againstthe conical wall of the cavity. The mounting means can be screwedcoaxially into the connecting mechanism at the upper end of theconnecting mechanism and, when the device is locked, press against thelongitudinal member, which is inserted in the transverse channel formedin the connecting mechanism, which in turn, presses on the clampingmeans. One disadvantage with this device is that, because the convex,front end of the clamping means is pressed into the conical cavity atthe screw head of the bone screw, there is a wedging effect whichcreates a levering force, which aligns the bone screw parallel with thecentral axis of the connecting mechanism.

Thus, there remains a need for a connecting mechanism that provides areduced overall height and which overcomes the problems associated withthe prior art. The present invention addresses these problems and othersby providing a device, which comprises a spherically shaped connectionbetween two members, the mechanism comprising clamping means, the frontend of which contacts the surface of a depression formed in the jointhead of the bone-fixation means, as a result of which a smaller overallheight of the connection mechanism is possible, and which, in theunlocked state, permits polyaxially angular position of the central axesof the two parts relative with respect to each another.

Pursuant to the invention, this objective is accomplished with a devicefor the ball-and-socket type connection of two members. The advantages,achieved by the invention, are a result of the fact that:

-   -   due the coincidence of the center of the joint and of the center        of the spherically shaped contact surface at the front end of        the clamping means and/or of the depression at the joint head,        locking of the two members with respect to one another at        different angles of the central axes of the two members becomes        possible without the occurrence of levering forces associated        with the prior art; and    -   because the joint head is fixed in the cavity by the clamping        means, the front end of which can be brought into contact with a        depression at the joint head, the latter can be configured        lower, as a result of which the device has smaller dimensions.

As will be described herein, the clamping means in various, differentembodiments can be configured differently at their front end, dependingon the desired form of contact with the depression, for example:

-   -   by configuring the front end of the clamping means as a point,        which is concentric with the central axis, a punctiform contact        between the front end and the depression can be achieved;    -   by configuring the front end of the clamping means as a        flattened surface, orthogonally to the central axis, linear        contact between the front end and the depression can be attained        and    -   by configuring the front end of the clamping means spherically,        a contact surface can be established between the front end and        the depression.

In further embodiments, the depression formed in the joint head of thebone-fixation means may be spherical or conical, as a result of which,by pairing clamping means with a spherically formed front end, a contactsurface or a linear contact can be attained between the front end of theclamping means and the depression.

In further embodiments, the depression is provided with a circular edge,which is concentric with the central axis and, together with thespherically configured front end of the clamping means, also forms alinear contact.

Preferably, the joint head of the bone-fixation means may be constructedrigidly, so that no expansion of the joint head is possible when thedevice is locked.

In a preferred embodiment of the invention, the device may include:

a) a first member whish is sized and configured as a bone-fixationmeans, for example, as a pedicle screw, a pedicle hook, etc., the lowersegment of which is used to fix the bone-fixation means to a patient'sbone; and

b) a second member which is sized and configured as a connectingmechanism and/or part for interconnecting the longitudinal member, forexample, a longitudinal spinal rod, with the bone-fixation means. Thesecond member may further include a channel, disposed transversely tothe central axis, for accommodating the longitudinal member and acentral borehole with internally formed threads, disposed coaxially withthe central axes.

Furthermore, the device may include mounting means for fixing thelongitudinal member, inserted into the transverse channel, so that thedevice can be used, for example, as a spinal fixation device.

The clamping means may directly lock and/or unlock, and may be in theform, for example, of a clamping screw, which may threadedly engage, forexample, the internal threads formed in the central borehole of theconnecting member. By this, it becomes possible that the locking and/orunlocking of the bone-fixation means by means of the clamping screw inthe connecting member can take place independently of the locking and/orunlocking of the longitudinal member by means of the clamping means.

On the other hand, the clamping means may indirectly lock and/or unlock,and coaxially move within the central borehole formed in the connectingmember and, for example, may be locked and/or unlocked by means of amounting means.

Alternatively, the transverse channel may open toward the side of thedevice, i.e., a side-loading pedicle screw. The side-loading pediclescrew may also include a covering cap, which may be pushed over theconnecting member parallel to the central axis. The covering cap mayinclude a tab formed parallel to the central axis, the tab enabling thechannel opening to be closed when the mounting means is tightened,thereby clamping the longitudinal member inside the channel between thewall of the channel and the tab. In this embodiment, with respect to thecentral axis, the channel preferably is offset from the channel openingso that it becomes possible that the clamping screw may be screwed intothe connecting member adjacent to the channel, so that, in comparison tothe previous embodiments, the clamping screw can be screwed into theconnecting member below the channel, thereby reducing the overall heightof the connecting member even more.

In yet another, different embodiment of the invention, the channel maybe configured at the upper end of the connecting member, i.e., atop-loading pedicle screw. The mounting means may be pressed directlyonto the longitudinal member, so that the longitudinal member can befixed in the channel by means of the clamping means.

In a further embodiment of the invention, the joint head of thebone-anchoring means may include two flat spots, which are parallel tothe central axis and which are separated by a distance B from oneanother, wherein the connecting member further includes a groove,provided parallel to the central axis, having a width W. The grooveproviding an enlarged opening of the cavity at the lower end of thesecond member perpendicularly to the width W in such a manner, that thejoint head, with the flat spots parallel to the side walls of thegroove, can be introduced into and removed from the cavity starting fromthe lower end of the second member. On the other hand, when the flatspots are rotated with respect to the side walls of the groove, thejoint head, due to the support in the cavity, is secured axially againstthe lower end of the second member.

BRIEF DESCRIPTION OF THE DRAWINGS

To facilitate an understanding of and for the purpose of illustratingthe present invention, exemplary and preferred features and embodimentsare disclosed in the accompanying drawings, it being understood,however, that the invention is not limited to the precise arrangementsand instrumentalities shown, and wherein similar reference charactersdenote similar elements throughout the several views, and wherein:

FIG. 1 shows a longitudinal section through an embodiment of theinventive device,

FIG. 2 a shows a section of the clamping means of an embodiment of theinventive device,

FIG. 2 b shows a section of the clamping means of a different embodimentof the inventive device,

FIG. 3 shows a longitudinal section through a further embodiment of theinventive device,

FIG. 4 a shows a section through the joint head of an embodiment of theinventive device,

FIG. 4 b shows a section through the joint head of a differentembodiment of the inventive device,

FIG. 5 shows a section through a further embodiment of the inventivedevice,

FIG. 6 shows a view from above of the embodiment of the inventivedevice, shown in FIG. 5,

FIG. 7 shows a view from below of the embodiment of the inventivedevice, shown in FIGS. 5 and 6,

FIG. 8 shows a section through a further embodiment of the inventivedevice,

FIG. 9 shows a section once again through a further embodiment of theinventive device,

FIG. 10 shows an exploded representation of the embodiment of theinventive device shown in FIGS. 5, 6 and 7 and

FIG. 11 shows an exploded representation of a part of an embodiment ofthe inventive device together with the fastening device.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to an exemplary, non-limitingembodiment illustrated in the figures and specific language will be usedto describe the same. It will nevertheless be understood that nolimitation of the scope of the invention is hereby intended, suchalterations and further modifications, and such further applications ofthe principles of the invention as illustrated herein being contemplatedas would normally occur to one skilled in the art to which the inventionrelates.

As shown in FIG. 1, the device includes a first member 1 and a secondmember 2, wherein the second member 2 includes a cavity 4 and clampingmeans 7 while the first member 1 has a central axis 5, an upper end 11,and a lower end 12. The upper end 11 of the first member 1 may include afirst joint segment 10, which may be constructed as a spherically shapedjoint head 3, the center of which, preferably lies on the central axis 5and forms a center of joint Z. As shown, the joint head 3 may beflattened at the upper end 11 of the first member 1. Furthermore, thejoint head 3 may include a depression 8, which may be partly sphericalwith a spherical shaped surface 9 and which terminates conically intothe upper end 11 of the first part 1. The spherical shaped surface 9 ofthe depression 8, having a center C, which preferably is constructed sothat center C lies on the central axis 5 and coincides with the centerZ.

The second part 2 may also have a central axis 6, an upper end 21, alower end 22, an upper segment 23 and a cavity 4, which is disposed atthe lower end 22 and forms the second joint segment 20. The cavity 4 maybe disposed coaxially with the central axis 6 and is tapered at thelower end 22 of the second member 2 by a constriction 14 formed on thelower end 22. Preferably, the constriction 14 has a spherical shapedsupport 37 forming a center which coincides with the joint center Z. Thespherically shaped support 37 being directed towards the interior of thecavity 4. Preferably, the spherically shaped support 37 is complementarywith the surface of the joint head 3, so that the two joint segments 10;20 can be rotated with respect to one another about three mutuallyperpendicular axes.

As shown, at the lower end 22 of the second member 2, the cavity 4 mayhave an opening 15, which may be coaxial with the central axis 6 andthrough which the lower segment 13 of the first member 1 can pass.

The clamping means 7 may be disposed coaxially with the central axis 6of the second member 2. The clamping means 7 may include a front end 16and a rear end 17 which are preferably disposed so that they may bedisplaced coaxially with the central axis 6. The front end 16 of theclamping means 7 may be constructed circularly cylindrically and may bebrought into contact with the spherically shaped surface 9 of thedepression 8. By pressing the front end 16 onto the spherically shapedsurface 9 of the depression 8, the joint head 3 can be locked in thecavity 4 formed in the second member 2. The first and second members 1,2 with the polyaxially angular arrangement of their central axes 5, 6being lockable because of the identity of location of the center C ofthe spherically shaped surface 9 of the depression 8 with the jointcenter Z.

As shown, in FIGS. 2a and 2b, in each case showing different embodimentsof the clamping means 7, the front end 16 of the clamping means 7, asshown in FIG. 2 a, may have a point which is coaxial with the centralaxis 6, whereas, as shown in FIG. 2 b, the front end 16 of the clampingmeans 7 is shown as being convexly spherical.

The device, as shown in FIG. 3, differs from the embodiment, shown inFIG. 1, only in that the depression 8 in the joint head 3 and the frontend 16 of the clamping means 7 are constructed differently. As shown,the front end 16 of the clamping means 7 is hemispherical with thecenter C lying on the central axis 6, and has a radius R. While thedepression 8 is conical with an annular elevation 38, which isconcentric with the central axis 6. The elevation 38 has a circular,sharp edge 39, which is directed towards the interior of the depression8. The axial distance between the edge 39 and the upper end 11 of thefirst part 1 and the diameter of the circular edge 39 are such that theedge 39 lies on an imaginary spherical surface with the center C lyingon the central axis 6 and with the radius R.

As shown in FIG. 4a, the joint head 3 differs from the embodimentsdescribed above only in that the depression 8 at the upper end 11 of thefirst member 1 has a conical, coaxial section 40 and an axiallyadjoining circular cylindrical section 41 with the diameter D at a depthT, measured from the upper end 11. The diameter D is selected so thatthe depression 8 is constricted at the depth T by the circularcylindrical section 41, so that a circular edge 39, concentric with thecentral axis 6, is formed by the transition between the conical section4 and the circular cylindrical section 41. Furthermore, the diameter Dand the depth T are dimensioned so that the circular edge 39 coincideson an imaginary spherical surface with the center C, lying on thecentral axis 6, and with the radius R, as was the case with theembodiment depicted in FIG. 3.

As shown in FIG. 4 b, the joint head 3 may have a conical depression 8forming a cone 42. The diameter of the cone 42 at the upper end 11 ofthe first member 1 and the conical angle of the cone 42 are dimensionedso that the cone 42 touches an imaginary spherical surface with thecenter C, lying on the central axis 6, and with a radius R, as was thecase with the embodiment depicted in FIG. 3.

A linear contact between the clamping means 7 and the depression 8 canbe achieved by a configuration of the depression 8 and of the front end16 of the clamping means 7 of one of the FIGS. 3, 4 a or 4 b.

As shown, in FIGS. 5-7 and 10, an alternate embodiment of the presentdevice is shown wherein the device comprises, a first member 1 havingbone fixation means 24, which as shown is a pedicle screw, but which mayalso be a pedicle hook, etc., and a second member 2 having a connectingpart 25 for connecting a longitudinal spinal member 26 to the bonefixation means 24. As partially shown, the bone fixation means 24 mayinclude a lower threaded shaft segment 13 and a joint screw head 3. Thelower surface side of the screw head, i.e., on the side facing the screwshaft, the screw head is preferably constructed with a partiallyspherically surface having a radius R₂, the center of the sphericallower surface, preferably lying on the central axis 6, forming the jointcenter Z.

The upper surface side of the joint head 3 preferably has a depression8, which has a partially spherical surface 9 with a center C and aradius R₁. The partially spherical surface preferably terminatingconically into the outer surface at the upper end 11 of the first member1. Preferably, the center C coincides with the joint center Z, so thatthe screw head can be locked in the connecting part 25 in such a mannerthat the central axis 5 of the first member 1 and the central axis 6 ofthe second member 2 can polyaxially rotate with respect to each another.

As shown, the cavity 4 in the connecting part 25, serving as a secondjoint segment 20, preferably includes a constriction 14, which as shownis a spherical support 37 having a joint center Z so that thespherically configured screw head is mounted on the support 37 so thatit can be rotated polyaxially about the joint center Z.

The device further includes a clamping means 7, which as shown in FIG.5, may be clamping screw having a rear end 17 with an external thread 47sized and configured to mate with complementary internal threads 31formed on the central borehole 30 of the connecting part 25. The rearend 17 of the clamping means 7 also may include means 44 foraccommodating a screwdriver, a hex-driver, etc. As shown, the front end16 of the clamping means 7 is formed with a spherical tip for contactingthe spherical surface 9 in the depression 8 at the screw head. Althoughother shaped tips are envisioned.

As best shown in FIGS. 7 and 10, the joint head 3 may be equipped withtwo flat spots 35, which are preferably parallel to one another and tothe central axis 5. The two flat spots 35 being separated by a distanceB from one another. Similarly, the cavity 4 may include a groove 36 ofwidth W, width W being greater than or equal to distance B, which isparallel to the central axis 6. The depth of the groove 36 is such thatthe opening 15 of the cavity 4 at the lower end 22 of the second member2 is enlarged perpendicularly to the width B, so that the joint head 3with the flat spots 35, parallel to the side walls of the groove 36, canbe passed into or out of the cavity 4 from the lower end 22 of thesecond member 2 and then, by rotating the first member 1 about thecentral axis 5, the flat spots 35 at the joint head 3 are twisted withrespect to the side walls of the groove 36, so that the joint head 3 canbe fixed axially against the lower end 22 of the second member 2 by theconstriction 14 formed in the cavity 4.

As shown, the device may further include a channel 27 for accommodatingthe longitudinal member 26, the channel being mounted in the connectingpart 25 with the channel axis 28 extending transversely to the centralaxis 6. As shown, the channel 27 may be opened laterally at theconnecting part 25 forming a channel opening 32 therein. The channel 27is provided at the connecting part 25 so that the channel axis 28 isdisplaced by a distance A away from the central axis 6 towards thechannel opening 32. A covering cap 33 is also provided, the cap 33 beingplaced coaxially on the central axis 6 over the connecting part 25 inorder to fix the longitudinal member 26 in the channel 27. The coveringcap 33 may include a tab 34, which is parallel to the central axis 6 andby means of which the channel opening 32 can be closed off by thecovering cap 33 so that the longitudinal member 26 inserted into thechannel 27 may be fixed in the channel 27. The covering cap 33 may bepressed coaxially along the central axis 6 by mounting means 29, whichare shown herein as a nut which may threadedly engage the externalthreads 43 formed on the upper end 21 of the connecting part 25 so thatthe longitudinal member 26, inserted in the channel 27, can be fixedwithin the channel 27. Alternatively, the connecting member 25 mayinclude internal threads for engaging an externally threaded nut.

Moreover, as shown, a device to prevent twisting can be mounted betweenthe connecting part 25 and the covering cap 33. This device may be inthe form of a flat spot 48 and a complementary constriction 50, the flatspot 48 being formed on a portion of the connecting part 25 extendingparallel to the central axis 6 while the constriction 50 is formed onthe central opening 49 of the covering cap 33, also extending coaxialwith the central axis 6. So that when the covering cap 33 is pushed overthe connecting part 25, the constriction 50 mates with the flat spot 48,so that twisting of the covering cap 33 about the central axis 6 withrespect to the connecting part 25 is prevented.

As shown in FIG. 8, the device differs from the embodiment, shown inFIGS. 1 and 3, only in that the second member 2, may include a channel27 formed in the connecting part 25 for accommodating a longitudinalmember, the channel 27 being open at the upper end 21 of the connectionpart 25. The clamping means 7 may be screwed coaxially along the centralaxis 6 against the internal threads 31 formed in the central borehole 30of the connecting part 25, so that the front end 16 of the clampingmeans 7 can be pressed against the surface 9 of the depression 8 at theupper end 11 of the first member 1. When the device is installed, theclamping means 7 is thus disposed between the longitudinal member 26,placed in the channel 27, and the cavity 4 in the second member 2. Thelongitudinal member 26 being locked in the channel 27 by mounting means29, which is shown as an externally threaded setscrew and which, afterthe longitudinal member 26 is placed in the channel 27, can be screwedfrom the upper end 21 of the connecting part 25 into engagement with theinternal threads 52. The internal thread 52 and the internal thread 31,both of which are disposed coaxially with the central axis 6 in thecentral borehole 30, can also be formed as a single internal thread.

As shown in FIG. 9, the device differs from the embodiment shown in FIG.8 only in that the internal thread 52 formed in the central borehole 30extends only so far from the upper end 21 of the connecting part 25, sothat a longitudinal member 26, placed in the channel 27, can be lockedin the channel 27 by means of the mounting means 29. As shown, thecentral borehole 30 is configured circularly cylindrically between thelongitudinal section of the central borehole 30, which is provided withthe internal thread 52, and the cavity 4, so that the clamping means 7can be moved parallel to the central axis 6 in the central borehole 30.At its rear end 17, the clamping means 7 may include a notch 51, whichextends transversely to the central axis 6. In the installed device, thelongitudinal member 26, placed in the channel 27, rests in the notch 51, so that, when the mounting means 29 is tightened, it presses againstthe longitudinal member 26, which, in turn, presses on the clampingmeans 7. The clamping means 7 thereafter being moved parallel to thecentral axis 6 against the front end, 22 of the connecting part 25thereby pressing with its front end 16 onto the surface 9 of thedepression 8 at the upper end 11 of the first member 1. Thus, tighteningthe mounting means 29, locks the longitudinal member 26 and the firstmember 1 simultaneously in the device.

As shown in FIG. 11, the device may include a second member 2 which isconfigured as a connecting part 25, clamping means 7 and a fasteningdevice 57. As shown, the fastening device 57 may include a sleeve-likeholder 58 and a threaded bolt 59, which can be moved coaxially along thecentral axis 6 of the holder 58. At its front end 63, the holder 58 hasa hexagon drive 62, which is parallel to the central axis 6, and whichis sized and configured to mate with a complementary hexagon socket 56formed on the rear end 21 of the connecting part 25. At its front end60, coaxially with the central axis 6, the threaded bolt 59 may includeexternal threads 61, so that the threaded bolt 59 may threadedly engagethe complementary internal threads 55 formed in the borehole 54, whichpenetrates from the rear end 17 of the clamping means 7 coaxially withthe central axis 6 so that the clamping means may be connected with thefastening device 57.

The clamping means 7, which, as shown, is provided with an externalthread 47 extending axially from the rear end 17, may threaded engagethe complementary internal threads 31 formed in the central borehole 30of the connecting part 25. Furthermore, in the rear end 17, coaxial withthe central axis 6, the clamping means 7 may include means 44 foraccommodating a screwdriver, a hex-driver, etc. As shown, the hexagonsocket may include a cross-sectional dimension, which corresponds to theexternal diameter of the internal thread 55. Similarly, the hexagonsocket 56, mounted at the rear end 21 of the connecting part 25coaxially with the central axis 6 in the central borehole 30, may have across-sectional dimension, which corresponds to the internal diameter ofthe internal thread 31 formed in the central borehole 30.

The present invention has been described in connection with thepreferred embodiments. These embodiments, however, are merely forexample and the invention is not restricted thereto. It will beunderstood by those skilled in the art that other variations andmodifications can easily be made within the scope of the invention asdefined by the appended claims, thus it is only intended that thepresent invention be limited by the following claims.

1. A device for the ball-and-socket joint type connection of two parts,comprising: a first member (1) and a second member (2), which areconnected with one another in the form of a ball-and-socket joint; thefirst member (1) having a central axis (5), a first joint segment (10)with an upper end (11) and a lower segment (13); the second member (2)having a central axis (6), a second joint segment (20) with a lower end(22) and an upper segment (23) with an upper end (21); the first jointsegment (10) being configured as a joint head (3) having a depression(8) at the upper end (11) whereas the second joint segment (20) isconfigured as a cavity, which accommodates the joint head (3), thecavity being concentric with the central axis (6) and having an opening(15) for the passage of the lower segment (13) of the first member (1)therethrough; wherein the cavity (4) further includes a constriction(14) formed by a tapered surface formed in the lower end (22) of thesecond member; the constriction (14) having a support (37) for the jointhead (3), which is directed towards the interior of the cavity (4);wherein at least one of the joint segments (10; 20) in the contactregion of the joint segment (10; 20) is configured spherically with ajoint center Z; wherein the joint-like connection between the two jointsegments (10; 20) is fixable by clamping means (7), said clamping meansincluding a front end (16) sized and configured to contact thedepression (8) formed in the first joint segment and a rear end (17) andcan be connected with the second joint segment (20); wherein at leastone of the depression (8) and the front end (16) of the clamping means(7) includes a partially spherical surface (9), which has a center Ccharacterized in that the joint center Z and the center C are identicalwhen the device is locked.